A couple things come to mind: 1. I'm amazed at how much data they make available (mainly, from a technical POV; for example, they have an open search API that you can use with JavaScript that can do real time searches) 2. The API is pretty tricky to navigate. It's doable, but boy oh boy, there are a lot of gotchas and the documentation is pretty lacking.
Would love to check out some other projects that make use of the Wikipedia API that people here may have made :)